1. Field of the Invention
The present invention relates to semi-trailer landing gear extension and retraction mechanisms, and more particularly to a pneumatic and hydraulic control mechanism that promotes coordinated extension and retraction of landing gear legs.
2. Description of the Prior Art
Many semi-trailers have front landing gear for support of the front of the trailer when the tractor is detached. Such landing gear generally has two spaced-apart, telescoping or Jack-type landing gear legs which extend downwardly from the bed of the trailer.
Traditionally, each leg is operatively attached to a screw and follower or a rack and pinion gear arrangement which is driven by a landing gear drive shaft extending between the legs. The drive shaft controls extension or retraction of the legs depending on the direction in which it is rotated. Most commonly, a hand operable crank is attached to the landing gear drive shaft. Manual rotation of the crank via a crank handle causes extension and retraction of the landing gear. Manually operated jack structures, however, are often difficult to use, require much time for their operation and expose the operator to potential injuries as he is positioned next to the trailer while turning the crank to raise or lower the landing gear.
More recently, landing gear systems have been provided with powered devices in order to raise and lower the landing gear. Some of these powered devices have been pneumatically powered. Typically, such pneumatic devices require a complex control and piping system to receive pressurized air for the motion actuators to rotate the crank shaft of the gear reduction assembly to raise and lower the landing gear. One class of prior art pneumatic devices has utilized pneumatic impact motors to drive the landing gear. Of course, those skilled in the art will understand that due to their pulsating drive mechanism, such pneumatic impact motors are not conducive to the desired smooth operation of raising and lowering the landing gear legs, especially in combination with gear reduction boxes.
Another class of prior art pneumatic mechanisms dispenses with the gearing system of traditional landing gear and utilizes a pneumatic bladder or chamber within each leg to telescope the landing gear.
Piston driven hydraulic landing gear devices are also known in the prior art. Similar to the pneumatic air bladder/chamber system referenced above, such devices replace traditional drive shaft/gearing systems and utilize hydraulic pistons to drive the landing gear. One drawback to hydraulic pistons is that they do not provide the same positive holding as provided by a gearing arrangement. Power failure or loss of fluid pressure could cause failure and collapse of the landing gear. As with the pneumatic systems, typically, such hydraulic devices require a complex control and piping system.
Moreover, with respect to either pneumatic or hydraulic bladder/chamber systems, it has been found that it is desirable to utilize two smaller pumps to provide pressurized fluid for actuation of the system. In the prior art, such pumps are typically located adjacent their respective jacks to pump hydraulic fluid from a reservoir located adjacent the jack to a chamber within the telescoping leg of the jack, and, as mentioned above, require complex control and piping systems. It has been found that such systems, in particular those requiring a significant amount of piping, couplings and the like, are readily subject to wear and/or damage by virtue of the operating environment to which they are exposed, i.e., high vibration, exposure to road debris, etc. Moreover, such systems have individual controls for the respective jacks. Such a control system promotes damage to the system and trailer frame, when the jacks are not correctly synchronized during raising and lowering, thereby causing twisting of the trailer frame.
In addition, such systems are generally large, cumbersome and permanently installed as an integrated part of the drive system, and do not lend themselves to retrofitting on trailers have pre-existing landing gear.